Heap leaching is one available option for the leaching of precious and base metals from, generally, low grade ores or waste materials. Biooxidation using a range of micro-organisms is one option for such a leach. Such biooxidation is variously described as the primary leach with an acid lixiviant or as a preconditioning step prior to, for example, a cyanidation step.
U.S. Pat. No. 6,207,443, assigned to Placer Dome, Inc., describes a process for the biooxidation of sulphide ores in which a previously biooxidised material is incorporated into a heap. A first portion of a feed material containing metal sulphides is biooxidised in a heap or vat to form a biooxidised fraction. That biooxidised fraction is subsequently combined with a second portion of the feed material, forming a combined feed material. That combined feed material may then be used to form a further heap. Biooxidation is performed in the further heap through the application of an inoculum comprising sulphuric acid and suitable microbes, with or without suitable nutrients. This lixiviant is allowed to percolate through the heap and the pregnant leach solution is removed at the base of that heap.
The manner in which the biooxidised residue from the first heap is combined with the second portion of the ore is by way of either the introduction of both ore feeds into the agglomerate or both may be placed directly onto a conveyor belt to the further heap. A further alternative is to place the biooxidised residue onto the top of a heap already formed from the second feed portion.
There are problems associated with combining the two feeds prior to the agglomerater as the conditions within the agglomerater are particularly harsh on the microbes present, particularly if the microbes must be present through the entire agglomeration process. The problem associated with the alternate versions of the combination of the two steams described above is that it is unlikely that all of the second stream or portion of ore is exposed to the microbes present in the biooxidised residue. Similarly, there is no mechanism by which it can be assured that a significant proportion of the second stream or portion of feed ore is exposed to the inoculum/lixiviant administered to the further heap. Such a heap is subject to the traditional problems associated with heap leaching, being, amongst other things, channelling.
U.S. Pat. No. 6,083,730, assigned to Geobiotics, Inc., describes a heap biooxidation process in which the heap is comprised of substrates, such as coarse ore particles, onto which a concentrate prepared from the ore to be biooxidised is coated. This coating occurs in an agglomerator prior to the stacking of the heap. The heap is then inoculated, in the typical manner, with bacteria capable of bio-oxidising the metal sulphide particles within the concentrate coated on the substrates. This inoculation occurs either during the stacking of the heap or immediately thereafter. This process seeks to expose as much metal sulphide to the lixiviant as possible through the fine coating of the substrates forming the heap. However, there is not described any mechanism by which the microbes for bio-oxidation are effectively evenly distributed throughout the heap.
In U.S. patent application Ser. No. 10/723,392 (Publication No. US 2004/0131520 A1) to Bruynesteyn, a process for the leaching of low sulphur content ores is described. This process includes a mechanism by which finely milled elemental sulphur is exposed to a culture of sulphur oxidising bacteria as a preconditioning step in which the sulphur particles are wetted and the bacteria attach themselves to those sulphur particles. The resulting combination of preconditioned sulphur, water and bacteria is then added to ore particles during a typical agglomeration process. This process focuses on the production of sulphuric acid during preconditioning, that sulphuric acid being used to partially satisfy the acid demand of the ore in the heap ultimately formed. Again, it is apparent that the sulphur oxidising bacteria are subjected to the entire agglomeration process and its harsh environment.
In U.S. Pat. No. 5,246,486 assigned to Newmont Gold Co. and Newmont Mining Corporation, there is described a process for the biooxidation of sulphides in a heap. One of the key components of this process is the formation of “particulates” from the sulphide ore particles. An inoculate is used in the formation of these particulates, the inoculum being described as sprayed on the ore whilst on the stacking conveyor, which process is indicated to reflect agglomeration as it is generally known. It is also noted in this specification that other processes may be employed for the particulate formation, including disc type agglomeration devices. Further, reference is made to dipping of ore into a liquid bath on a conveyor, and also to the use of screw extruders.
The particulates formed as described above are subsequently used in the stacking of the heap. It is suggested that the formation of a heap in this manner results in efficient leaching upon the subsequent dispersing of a leaching solution, with or without additional nutrients, through the heap in known manner. Whilst the distribution of the inoculum throughout the heap by way of formation of the particulates is advantageous, the microbes of the inoculum are again subject to severe physical conditions during agglomeration, including prolonged exposure to abrasive forces which may reduce their effectiveness in the heap ultimately formed.
In International Patent Application PCT/IB98/00969 (Publication No. WO 98/51827), to Echo Bay Mines Limited, an integrated tank/heap bio-oxidation process for use on sulphide ores is described. The process involves the splitting of a refractory sulphide ore source into two steams. The first of these streams is exposed to a sulphide digesting microorganism in a tank reactor for the acclimatisation of the microorganism to the particular sulphide ore, after which the partially digested ore is combined with the second stream of sulphide ore. It is subsequently solid/liquid separated (dewatered) and the solid portion, with or without an agglomeration step, used to construct a heap. The liquid from the solid/liquid separation step is then used to inoculate the heap and perform the biooxidisation process. Subsequent to the biooxidation step the heap is subjected to a lixiviation process, such as cyanidation. As noted, solids and liquids from the biooxidation reactor are passed in turn to the thickener, the content of which ultimately passes to solids/liquids separation. After separation, the entire content is effectively recombined in the heap leach. This creates a very step-wise or batch-like process which may introduce limitation as to how effectively the process may be practised. Further, the solids/liquids separation or dewatering step, in conjunction with the agglomeration step for the solids therefrom prior to stacking of the heap again exposes microorganisms to relatively harsh physical conditions. Also, the inoculum is subsequently applied to the heap in much the same manner as the majority of the prior art, by way of percolation through the heap.
As noted above, many patent specifications describe bio-oxidation processes in a heap leach environment, together with the inoculation of the ore and/or the heap. However, little or no description is provided with regard to the manner in which cultures of appropriate micro-organisms for bio-oxidation are produced, maintained, handled and transported in sufficient volumes for commercial heap leaching operations. This in itself is a difficult and complicated exercise.
A wide range of bacterial species have been described as suitable for, or present in, biooxidation processes of the type described above. These include Thiobacillus Ferrooxidans; Thiobacillus Thiooxidans; Thiobacillus Organoparus; Thiobacillus Acidphilus; Sulfobacillus Thermosulfidooxidans; Sulfolobus Acidocaldarius, Sulfolobus BC; Sulfolobus Solfataricus; Acidanus Brierley; and Leptospirillum Ferrooxidans. These species are generally described as indigenous to the ore to be biooxidised and the conditions of the leach are provided to allow the indigenous bacterial species to thrive and effect the biooxidation process.
The temperature at which the heap leach operates has an impact upon those bacterial species that will be most active in the biooxidation process. This often also impacts upon the efficiency/effectiveness of the leach. This in turn will affect the recovery of metal from pregnant leach solution in downstream metal recovery steps. Such issues are heightened when attempting to biooxidise traditionally difficult to leach ores, such as chalcopyrite ores. International Patent Application PCT/AU2004/001597 (WO 2005/056842) describes such a process, as does PCT/AU2004/000236 (WO 2004/081241), in which the heap leach is conducted at a temperature intended to allow certain bacterial species, introduced into the heap, to operate. The process of PCT/AU2004/000236 utilises the presence of mesophilic bacterial species to raise the temperature of the heap to a point at which thermophilic bacterial species will operate to biooxidise the chalcopyrite present. U.S. Pat. No. 6,110,253 also describes biooxidation in a heap leach for a chalcopyrite ore but uses an external source of heat, such as heated leach solution or the use of hot air or steam in the pipe work used to introduce oxygen to the heap.
The present invention has as one object thereof to overcome substantially the problems or disadvantages associated with the above prior art, or to at least provide a useful alternative by which those processes may be operated.
The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in either Australia or any other territory as at the priority date of the application.
Throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.